Enhanced vision systems (EVS), such as those used in aircraft, are used to detect infrared radiation or visible light emanating from a scene. In the case of infrared radiation, typical EVS include one or more detectors that detect short-wavelength infrared radiation (SWIR) and long-wavelength infrared radiation (LWIR). These systems process the SWIR and LWIR with use of a single channel or with separate short-wave (SW) and long-wave (LW) channels.
Typical EVS using multiple channels process and combine (or fuse) SWIR and LWIR through a light skimming approach. In this approach, the SW channel extracts peaks or local maxima of a SWIR signal to identify centers of SW radiation sources, such as runway lights. The peaks or local maxima are used to generate a SW video signal in which the peaks or local maxima are represented as a pattern of computer generated symbols, such as dots. The LW channel processes the LWIR to generate a LW video signal representing a background scene. Thereafter, the SW video signal is combined with the LW video signal to generate a final image—the only contribution to the final image from the SW channel being the pattern of computer generated dots. Examples of such multiple-channel EVS are described in U.S. Pat. Nos. 6,232,602; 6,373,055; 6,806,469; and 7,102,130.
Furthermore, typical multi-channel systems implement an iris control feature in which an average signal level of a SWIR signal is fed back to control an iris position of a SW detector. With this approach, the iris of the SW detector is controlled to maintain the average signal level of the SWIR signal at a preset level.
Typical multi-channel systems work well in original situational awareness applications. Shortcomings in typical multi-channel systems, however, become apparent in certain applications. For example, in approach and landing scenarios for aircraft, it is necessary to detect runway lights more quickly and in worse atmospheric conditions. As the SW channel of a typical multi-channel system is operated at higher gain to achieve earlier detection, image artifacts in the form of extreme blooming of the runway lights are displayed. This light blooming can be so extreme as to obscure any useful image of the runway environment. Thus, a need exists for a multi-channel enhanced vision system that performs well under all conditions, including those in which the SWIR gain is set very high to allow early detection of SW radiation sources. A need also exists for a multi-channel system that is capable of displaying fine image detail that includes contribution from very sensitive SWIR.